A conventional fuel vapor processing system for processing fuel vapor produced from an automotive fuel tank is illustrated in FIG. 6, Such a system is disclosed for instance in Japanese patent application No. 2002-57054. In this system, an upper part of a fuel tank 1 communicates with a canister 2 via a fuel vapor passage 3. In this case, the upper part of the fuel tank 1 consists of two levels. The fuel tank end of the fuel vapor passage 3 is branched into a first branch passage 3a that communicates with the lower level of the fuel tank upper part and a second branch passage 3b that communicates with the upper level of the fuel tank upper part. The lower level of the fuel tank upper part is provided with a float valve 4 that communicates with the first branch passage 3a, and the upper level of the fuel tank upper part is provided with a cut valve 5 that communicates with the second branch passage 3b. The fuel tank 1 is additionally provided with a fill pipe 9 for conducting fuel from a fill nozzle G of a fuel pump into the fuel tank 1.
The float valve 4 comprises a valve member 4a that starts floating on the fuel surface when the fuel tank 1 is filled nearly to full, and a port 4b provided at the corresponding end of the first branch passage 4a is closed by the valve member 4b when the fuel tank 1 is full. The cut valve 5 comprises a valve member 5a that floats on the fuel surface when the fuel tank 1 has tilted to a certain extent, and a port 5b provided at the corresponding end of the second branch passage 4b that is closed by the valve member 4b when it floats.
A check valve 21 is provided in an intermediate point of the second branch passage 3b. When the fuel tank 1 is filled to full and float valve 4 has closed the first branch passage 3a, this check valve 21 permits the internal pressure of the fuel tank 1 to rise to such an extent that the fuel level in the fill pipe 9 rises and activates a sensor of the fill nozzle G to automatically stop the supply of fuel from the nozzle G. When the internal pressure of the fuel tank 1 has risen beyond a prescribed level owing to the additional rise in the fuel surface level, the check valve 21 opens to conduct the fuel vapor to the canister 2 and prevents the fuel vapor from escaping out of the fill pipe 9.
In such a fuel vapor processing system, because the rise in the internal pressure of the fuel tank 1 when the fuel tank 1 is filled full is relative small, the opening pressure of the check valve 21 is set relatively low so that the fuel vapor in the fuel tank 1 when it is filled full may be conducted to the canister 2 via the check valve 21, and absorbed by charcoal or other material filled in the canister 2. Thereby, when filling fuel into the fuel tank, the fuel vapor is prevented from escaping to the atmosphere via the fill pipe 9.
Even when fuel is not being filled into the fuel tank 1, the internal pressure of the fuel tank 1 may rise if the fuel tank 1 is placed in a high temperature environment. In such a case also, the check valve 21 opens and allows the fuel vapor in the fuel tank 1 to be absorbed by the canister 2. When the surrounding temperature is high, a significant amount of fuel vapor can be produced. Therefore, the opening area of the check valve 21 is required to be large enough to accommodate a large flow rate of fuel vapor resulting from such an event.
However, some of the existing fuel fill nozzles are equipped with a sensor that detects the rising of the froth or foam on the fuel surface in the fill pipe 9 to detect the tank full state and automatically stop the filling of fuel. When the opening area of the check valve 21 is increased as mentioned above, the opening of the check valve 21 may so rapidly stop the rise in the internal pressure of the fuel tank 1 that the froth fails to rise in the fill pipe and the timing of detecting the tank full state may be delayed when a fuel nozzle equipped with such a sensor is used. Such a delay in detecting the tank full state may lead to spilling fuel.